A proton initially has v=(-7.00)i+(-18.0)j+(-5.50)k and then 3.70 s later has v=(5.50)i+(-18.0)j+(4.90)k (in meters per second). (a) For that 3.70 s, what is the proton's average acceleration aavg in unit vector notation, (b) in magnitude, and (c) the angle between aavg and the positive direction of the x axis?